Leadership and necromancy: Characters who animate and have the leadership feat can substitute follower slots to increase the number of undead they control. (Since making this I discovered the Undead Leadership feat in Libris Mortis, which kind of renders this obsolete.)

 

In addition, clerics who channel positive energy can animate positive energy spirits (like those in Order of the Stick) which they can bolster and command, but negative energy clerics can turn or destroy.